Analog to digital converter ("ADC") devices are well known. Generally, these devices periodically sample the instantaneous analog input signal and divide it into parts over a resistor network to produce a binary representation of the instantaneous analog signal. The analog waveform is thus converted into a series of binary values, i.e., a digital signal, at the selected sampling rate. The sampling rate determines how accurately the digital signal represents the analog signal. The number of bits used in the binary representation determines the amplitude resolution of the digital signal. Practical restraints and economics limit the number of bits that may be efficiently used.
Importantly, the dynamic range of the analog signal to be converted must be adjusted to fit within the input range of the ADC. The term "dynamic range" refers to the potential difference between the positive and negative peaks of the analog signal, whether or not the signal is bi-polar. Adjusting the dynamic range conventionally involves DC level shifting the analog signal and adjusting the gain of the analog signal. However, when the dynamic range of the analog signal changes over time or in response to monitored parameters, e.g., changing signal strength, it is necessary to adjust again the new dynamic range for conversion by the ADC.
One known technique for solving this problem, as illustrated in FIG. 1 of the drawings, monitors the peak of the analog signal and adjusts the gain of the analog signal in response to the measured peak of the analog signal to a level that falls within the input range of the analog to digital converter ("ADC"). The digitized signal is then provided to a device such as a microprocessor for subsequent processing. For bi-polar analog signals that are to be converted by a unipolar ADC, the DC level of the gain adjusted analog signal may be shifted to provide a unipolar input to the ADC.
U.S. Pat. No. 4,827,191 refers to an analog to digital convertor circuit that includes an ADC having a controllable input range that is defined by a maximum span input and a minimum span input and a peak detection circuit for detecting positive and negative peaks of the analog signal. Signals representative of those detected peaks are provided to the maximum and minimum span inputs, respectively. The '191 patent also refers to using a positive peak detector and circuit for clamping the negative peaks of the analog signal and the negative span input of the ADC to ground. Accordingly, the circuit converts the analog signal relative to the maximum and minimum span inputs so as to provide an input range adjustment for a digital conversion of the entire analog signal.
It is known to use ADC devices in connection with magnetic ink character recognition techniques. Industry convention has established a font style for printing characters using magnetic ink. This convention provides that each character, when read by a suitable magnetic transducer, will produce a unique signature as an analog, pulsatile signal waveform having from four to eight peaks of one or more amplitudes.
One of the problems with analog to digital conversion in magnetic ink character recognition is that the quality of ink, and thus the intensity of the magnetic field presented to the transducer, may vary from character to character and from document to document. In addition, the mechanics of rapidly moving the transducer relative to characters printed on a document may result in spatial variations between the transducer and the ink. These variations may be caused by the transducer head lift off or force lift off, or as a result of other moving parts causing undesired vibrations, such as worn idler wheels. Consequently, the sensed analog signals have a dynamic range and magnitude that may vary from character to character and/or from document to document by as much as 25:1.
The technique illustrated in FIG. 1 may be used to adjust the gain of the analog signal for analog to digital conversion to account for such signal variations. However, this technique introduces an undesirably high count of components to adjust adequately the gain of the analog signal within the maximum and minimum limits of the ADC. These components consume valuable space in a machine and incremental amounts of power, and also provide a potential source of part failure, assembly error, and circuit instability.
The technique described in U.S. Pat. No. 4,827,191 will not overcome these problems. For one thing, the reference span inputs follow the detected peaks and thus normalize the analog signal relative to a continuously changing reference signal. This has the effect of suppressing or losing the information represented by different amplitude peaks in an analog signal waveform, and in particular, a signature waveform. Another problem with this technique is that it is apparently susceptible to interpreting noise peaks as valid signal information and normalizing the signal accordingly. This may result in erroneous digitized signals.
There is thus a continuing need for improved analog to digital converter circuits and in particular a need for such circuits for converting analog signal waveforms containing amplitude modulated information, e.g., signature waveforms representing magnetic ink characters for document processing.